Restricted Hartree-Fock and unrestricted Hartree-Fock as reference states in many-body perturbation theory: a critical comparison of the two approaches |
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Authors: | Petr Čársky Ivan Hubač |
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Affiliation: | (1) J. Heyrovský Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Dolej kova 3, 182 23 Prague 8, Czechoslovakia;(2) Department of Biophysics and Chemical Physics, Faculty of Mathematics and Physics, Comenius University, 842 15 Bratislava, Czechoslovakia;(3) Present address: Max-Planck-Institut für Physik und Astrophysik, Institut für Astrophysik, Karl-Schwarzschild-Str. 1, W-8046 Garching b. München, Federal Republic of Germany |
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Abstract: | Summary The paper deals with two topics related to the problem which reference state is better for many-body perturbation theory: restricted Hartree-Fock (RHF) or unrestricted Hartree-Fock (UHF)? The first topic concerns the potential surfaces. Several examples are presented to show shortcomings of the two approaches and a simple way is presented which seems to give a useful potential curve in the whole range of interatomic distances by a composition of RHF and UHF potential curves. The second topic concerns the many-body perturbation theory for open-shell systems in the RHF formalism. The method is critically examined and compared with the ordinary many-body perturbation theory using UHF as the reference. This examination of many-body techniques provides also some insight into the problems inherent of the SCF theory: spin contamination from higher multiplets, localization of orbitals, and self-consistency effects. |
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Keywords: | Many-body perturbation theory Hartree-Fock Potential surfaces Open-shell systems |
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